1. Technical Field
The invention concerns a sensor with a one-layer or multi-layer sensor element operating in a contact-free manner and a housing comprising an electric/electronic connection and, in given cases, electronic components, where the sensor element comprises a coil arrangement whose windings have a defined line width, line thickness, and line spacing per layer and whose layers have a defined layer thickness and, in given cases, a defined layer spacing.
Furthermore, the invention concerns a corresponding sensor element, namely for use in a sensor according to the invention.
2. Description of Related Art
For measuring distance and monitoring position inductive sensors or eddy current sensors are frequently used. An essential component of the two types of sensor is a coil which is supplied with alternating current. In traditional sensors the coil regularly consists of numerous windings of insulated copper wire which are fixedly connected to one another with self-bonding wire or other sealing technologies and, depending of the type of sensor with or without a ferrite core, are integrated into the housing. In the housing or also offset therefrom electronics are provided which comprise an oscillator and the demodulator. In connection with this the coil is part of an oscillating circuit which is either a component of the oscillator or is supplied by it. Important in connection with this is high quality of the oscillating circuit in order specifically to ensure high measurement sensitivity of the sensor. High measurement sensitivity is the basic prerequisite for a wide range of measurement with simultaneously low susceptibility to interference.
For a rather long time sensors have been known in which the coil is implemented as a flat coil in printed circuit board technology. The printed circuit board technology has the advantage with respect to wound coils that the production costs are lower and the coil is part of an electronic printed circuit board. Recently the flat coil technology has been extended to ceramic substrates. Therein the coils are applied in the form of conductive printed circuit layers on a ceramic substrate. Several substrates are stacked one over another in layers, where the electrical connection between the layers is produced by through-connections. The layers are connected to one another by a sintering process at high temperatures and after the sintering form a compact unit.
An increase of the range of measurement of the sensor is possible due to the fact that the base distance between the sensor and the object to be measured is reduced. This can be achieved by the coil element being formed as part of the housing. The reduction of the base distance is achieved by the absence of the all-metal construction or the cap customary in conventional sensors. By means of a tight metal-ceramic connection between the coil element and housing there is in addition the advantage of a possible hermetic sealing of the interior of the sensor.
Along with the aforesaid advantages of the ceramic coils there are however also technical limitations. In particular for small coil diameters for small ranges of measurement conventionally wound coils exhibit, for example, significantly better quality, that is, higher inductances with lower resistances. For the quality of an oscillating circuit the following applies:
  Q  =            ω      ⁢                          ⁢      L        R  whereQ: coil qualityL: inductanceω: angular frequencyω=2πff frequency of the coil currentR: coil resistance
An increase of the inductance alone is however not expedient since with too high an inductance capacitive effects reduce the sensitivity of measurement or the time constant (and thus the temporal resolution) of the sensor will even become too high. Thus for a given measurement frequency an optimal inductance and an ohmic resistance must be chosen with which as high a quality as possible can be achieved.
For traditional wound coils the volume ratio of current-carrying copper litz wire to the insulating components lying therebetween (wire insulation and air or potting compound) is very large. In the optimization the wire cross section, the length, and the diameter of the coil as well as the number of windings can be affected. Thus it is possible over wide ranges to adapt the inductance as well as the ohmic resistance nearly independently. This leads to higher sensitivities in comparison to ceramic coils.
There the ratio between current-carrying layers and the insulating ceramic substrate is unfavorable. Furthermore, the ceramic substrate is only available in certain thicknesses. Also, the application of the current-carrying layers, e.g. by thick-layer printing processes, permits only certain ratios of printed conductor width to printed conductor thickness. Thus, for example, in thick-layer printing the width of the printed conductor must, depending on the paste system used, be clearly greater than the thickness.
An additional factor in coil design is the capacitive coupling of the windings among themselves. For coil systems in ceramic substrates the capacitive coupling is generally greater than for wound coils since despite greater distance of the printed conductors from one another also the effective surface between the printed conductors is greater and in addition the dielectric constant of the ceramics of ∈≈8 increases the capacitance.
A known technology for achieving, in comparison to the screen printing process, as small a layer spacing as possible with a large line thickness is embossing. With an embossing die at the position of the individual windings the individual layers of the ceramic substrate are deformed.
After the embossing the conductive winding must be applied. This can, for example, be done by a photochemical process such as the FODEL® technology of DuPont or with a suitable printing process, e.g. by screen printing.
In order to be able to realize structure sizes which are as small as possible in a manner which is reliable in processing, a corresponding embossing tool must be manufactured very precisely. For different layer geometries an individual embossing tool is needed. These two prerequisites make the embossing very time-consuming and expensive. In embossing there is no removal of material. Rather, the individual layers are deformed. The therewith associated expulsion of material leads to different density distributions in the material which can create problems in further processing. Thus, for example, during sintering of the ceramic stresses in the material arise.
A photochemical process, such as, for example, FODEL, requires many process steps well coordinated with one another. As a rule a photochemical process comprises the process steps printing with photopaste, illumination, development, and washing.
The process steps needed in addition, in comparison to the standard process, as well as tools and consumable materials, are elaborate and expensive. Furthermore, at the edges of the photosensitive layer smearing can occur. This is particularly critical for small structural sizes since thereby short circuits between the windings can occur.
The printing of an embossed layer proves to be difficult above all for relatively small line spacings and widths since the positioning of the paste over the embossing must be done precisely. With imprecise positioning there arises at the edge of the winding smearing which after sintering can lead to short circuits.
Through the optimization of the coil design with the aid of embossing technology and with relatively high costs and great expenditure of time only limited improvements of the sensor characteristics can be achieved. Due to the greater number and as a rule also more elaborate process steps the reproducibility and the reliability of the process are reduced.
An additional possibility for production consists in removing material directly at the position of the windings. This can, for example, be done with a laser. Laser processing is however relatively time-consuming. This makes the process per se elaborate and expensive. In laser processing material is removed directly so that no compacting of material occurs. However, in so doing the faces of the cut are very rough. The great roughness creates problems during later filling with paste. Thus, for example, small air bubbles can arise which at the high sintering temperatures lead to stresses in the material due to the compressive expansion. Furthermore, the roughness enlarges the surface of the conductive layer. Since due to the measurement principle high frequencies are required, as a result of the skin effect the current flows through the coil substantially at the surface, whereby ultimately the resistance ultimately increases. The resistance also increases if due to the roughness at the boundary surface material mixtures between the conductive paste and the insulating ceramic occur. These effects have a negative effect on the electrical data of the sensor.
Sensors of the generic type, in particular of multi-layer ceramics, have been known for a rather long time. Merely by way of example let reference be made to DE 10 2008 016 829 A1 and DE 103 14 875 A1.